You need to be between 18 and 60 years old to be eligible for Critical Illness insurance. However, once you have purchased the Critical Illness rider, you can renew it (and be covered) until you are 69 years old.

Yes. First, the claim has to be one of 39 Critical Illnesses.There are very specific definitions as to what is considered to be one of these Critical Illnesses.
Second, you must be diagnosed with Critical Illness within the coverage period.
Third, you are allowed only one Critical Illness claim per Term Life policy.
Fourth, there is a waiting period of either 30 or 60 days, depending on the type of Critical Illness, before you can claim.
Finally, there is a list of exclusions from coverage of Critical Illness. These are summarised in the Product Disclosure Sheet and listed in the policy document.

A waiting period is when you are not allowed to make a claim for critical illnesses diagnosed or occurring within a certain period after you have bought our policy. For Heart Attack, Coronary Artery By-Pass Surgery or Cancer, the waiting period is 60 days. For all other Critical Illnesses, the waiting period is 30 days.

In addition to the waiting periods for certain Critical Illnesses, there are certain situations that are not covered by your Critical Illness insurance. For example, if your Critical Illness was caused by AIDS or an HIV infection, alcohol or drug abuse or congenital conditions that was diagnosed before your 6th birthday, For the full list of exclusions, please see the Product Disclosure Sheet.

This is likely due to some factor in your health and lifestyle profile that makes you ineligible. However, if you purchased a Fi Term Life policy, your fundamental life insurance needs will be covered.